Return beam holography

ABSTRACT

Method and apparatus for constructing and reconstructing a hologram, whereby the hologram is reconstructed upon the original image plane used in constructing the hologram, by use, in one embodiment, of a spherical mirror placed to reflect divergent light originally passing through the hologram, back through the hologram as convergent light. Included are liquid gate means for reducing reflection glare from the hologram onto the reconstructed image.

United States Patent Inventors Appl. No.

Filed Patented Assignee Einar S. Mathisen Poughkeepsie;

Leonard S. Sheiner, Wappingers Falls, both of, N.Y.

Oct. 2 1 1969 Sept. 14, 1971 international Business Machines CorporationArmonk, N.Y.

RETURN BEAM HOLOGRAPHY 13 Claims, 7 Drawing Figs.

US. Cl

int. Cl...v

Field of Search References Cited UNITED STATES PATENTS 5/1970 Brooks eta1 OTHER REFERENCES Denisyuk, Optics & Spectroscopy, Vol. 15, No. 4, pp.279- 284 (10/1968).

Casler et al., Applied Physics Letters, Vol. 10, No. 12, pp. 341- 342(6/1967).

Brandt, Image Plane Holography 14211429 (7/1969) 350-35 PrimaryExaminer- David Schonberg Assistant ExaminerRobert L. ShermanAttorneys-l-lanifin and Jancin and Melvyn D. Silver 8 Applied OpticsABSTRACT: Method and apparatus for constructing and reconstructing ahologram, whereby the hologram is reconstructed upon the original imageplane used in constructing the hologram, by use, in one embodiment, of aspherical mirror placed to reflect divergent light originally passingthrough the hologram, back through the hologram as convergent light.Included are liquid gate means forreducing reflection glare from thehologram onto the reconstructed image.

RETURN IBIEAM HOILOGRAIPHY FIELD OF THE INVENTION Method and apparatusfor the construction and reconstruction of holograms. Included are thenecessary lens systems and apparatus for reconstructing the hologramupon its original image plane in original size and orientation.

BACKGROUND OF THE INVENTION Hologram optics, first described by Gabor in1948, have become practical only with the advent of the laser, whichprovides the intense, phase-coherent source needed to easily record andview holograms. The unique properties of holograms stem from the factthat they record both phase and amplitude. This eliminates the need torecord focused images. Lenses are not strictly necessary either forrecording or viewing. The reconstructed wave front proceeds onward fromthe hologram as if it emanated from the original reflecting object.

Each area of a hologram taken by diffused transmission or diffusedreflection, however small, contains information of the entire scene. Anypart of the hologram can be illustrated to reconstruct the entireoriginal scene. However, resolution of the reconstructed image isdegraded as the area of the illuminated hologram is decreased. But sincethe information is distributed equally over the entire recording area,holograms are relatively insensitive to breakage, scratching, and dustparticles.

Thus, the basic properties of holograms would appear to lend themselvesto microcircuit manufacturing applications, particularly wherephotosensitive resists are used as a step in the manufacturing processof microcircuit elements. Current glass masks used with conventionaloptical systems in the photorcsist step in the manufacture ofmicrocircuit elements have basic inherent defects. A current techniqueis to expose a photorcsisted area on a microcircuit element such as asemiconductor wafer, through a glass mask, which is in contact with thephotoresist surface. This leads to excessive wear and degradation of theglass mask pattern. Further, whether the mask be metal, or aphotographic plate or a photographic film, the continuous handling ofthe mask tends to degrade the image recorded in the film, glass, ormetal mask. Should a part of the image on the mask be injured, thatinformation is irretrievably lost.

While holograms have the advantage of retaining all information uponevery part of the hologram, a practical manufacturing technique isrequired to successfully use holographic techniques to exposephotoresist surfaces. Preferably, these techniques should allow thehologram to be made from the highly accurate master mask, and allow thereconstructed hologram to be focused onto the photosensitive surface inexactly the same size and alignment as the original master mask.Further, any such noncontact" exposure should avoid glare or foggingeffects, which will affect resolution of the photosensitive surfacebeing exposed on, for example, a microcircuit element. Since a greatnumber of microcircuit elements must be serially exposed to allow aneconomic manufacturing system, the alignment means for the microcircuitelement must be sure and simple.

Prior holographic systems have often utilized an excessive number oflenses, or a complex alignment means, none of which lend themselvestoward economic utilization of holographic effects toward themicrocircuit electronic industry.

Thus, an ideal system would be one allowing the construction of ahologram in the same apparatus in which the reconstruction of thehologram will occur. If the reconstruction of the hologram, in turn, canbe made to occur directly on the object plane from which the hologramwas initially constructed, then size and alignment problems areinherently overcome.

SUMMARY OF THE INVENTION Thus, an object of this invention is to allowhigh resolution holographic reconstruction on the original image planeused to make the original hologram.

A further object is to allow such reconstruction without the use ofexpensive lens systems.

Another object is to eliminate glare from the hologram from interferingwith the reconstructed image.

Another object is to provide a holographic reconstruction system usefulin microelectronic circuit manufacture.

These and other objects are met by the holographic construction andreconstruction system of this invention. Briefly stated, in oneembodiment, reconstruction of a hologram on the original object planefrom which the hologram was constructed is achieved by placing thehologram in the original position occupied by a photographic plate fromwhich the hologram is made; illuminating the hologram with a divergingreference beam identical to the original reference beam from which thehologram was made; and placing a spherical mirror behind the hologram toreflect the diverging beam passing through the hologram back through thehologram as a converging beam. The spherical mirror has the property ofand is so aligned such that the radius of curvature of the mirror iscoincident with the origin point of the diverging beam.

In another embodiment, glare caused by reflection from the front surfaceof the hologram that may interfere with or affect the resolution of thereconstructed image is eliminated by use ofa liquid gate between theorigin point of the diverging beam and the hologram itself.

These, and other embodiments utilizing plane and convex sphericalmirrors, will best be understood when read in conjunction with thefollowing drawings and general description.

IN THE DRAWINGS FIG. I shows a schematic of the apparatus for theconstruction ofa hologram, utilizing a divergent reference beam.

FIG. 2 shows a schematic of the apparatus of FIG. 1, adapted for thereconstruction of the hologram constructed by the apparatus of FIG. ll.

FIG. 3 shows an embodiment of a liquid gate to eliminate glare inreconstructing a hologram by the apparatus of FIG. 2.

FIG. 4 shows a schematic of the apparatus for the construc tion ofahologram utilizing a convergent reference beam.

FIG. 5 shows a schematic of the apparatus of FIG. 4, adapted for thereconstruction of the hologram constructed by the apparatus of FIG. 4.

FIG. 6 shows a schematic of the apparatus of the construction of thehologram utilizing a parallel reference beam.

FIG. 7 shows a schematic of the apparatus of FIG. 6, adapted for thereconstruction of the hologram constructed by the apparatus of FIG. 6.

GENERAL DESCRIPTION FIGS. 1 and 2 show the preferred embodiment of thisinvention.

FIG. I is a schematic showing the construction mode of the holographicconstruction-reconstruction apparatus of this invention. FIG. 2 showsthe reconstruction mode.

The hologram of an object, such as the master mask for integratedcircuit manufacturing processes, is constructed by the method and withthe apparatus as shown in FIG. 1. An incoming light beam I is dividedinto two parts by beam splitter 2. The reference portion 3 of the beamenters a first focusing means 18, where it is reflected by mirror 4 tolens 5, which acts as a means for creating a divergent reference beam 6.Located within reference beam 6 is a photographic plate 7, held inposition by a photographic-holographic holding means, not shown. Apinhole 8 is utilized, as multiple reflections and aberrations of theoptical components can give rise to nonuniform wave fronts, which canseverely degrade the quality of the resultant hologram. As both thereference beam and the illuminating beams are often made divergent bypassing them through microscope objectives or lenses, a 5-10 micronpinhole, for example, at the focal plane of the objective lenses 5 andIll eliminates most of the effect caused by multiple reflections andpoor quality in the preceding optics. Placing the pinhole as close aspossible to the output end of the optical system eliminates distortionsimposed by preceding beam splitters and lenses.

The other half of the incoming beam enters a second focusing meansgenerally shown as 10. This system comprises a lens ll 1, pin hole 12serving the same function as pinhole 3, lens 13, mirror 14, and mirror15. The incoming light passes through the system as shown in the figure.A diffuser 16 is placed just before the object 17 from which thehologram is to be made, in this case, a master mask. The object 17 islocated in place by an object hold means, which may be any means forholding the object in place while not interfering with the image on theplate. A common photographic plate holder will often be sufficient, aswould an XY alignment table.

A diffused beam of light emanating from the master mask will impingeupon photographic plate 7, completing the holographic exposure. Upondevelopment of the photographic plate 7, the image formed is thehologram, which is used in reconstruction.

The 6,328A helium-neon laser or any of the other commonly used means forcreating monochromatic coherent light may be used as a light source. Theindicated helium-neon laser is readily available, stable, and easy tooperate. The type of lasers useful in the making of holograms is furtherdiscussed in How to Make Laser Holograms," by K. S. Pennington,Microwaves, Oct. 1965, page 35. The wavelength of the light used should,of course, be suitable for later photoresist or photosensitive materialexposure.

Kodak 649F Spectroscopic plates are most commonly used in themanufacture of holograms. The manufacturer specifies the resolution ofmore than 2,000 lines per millimeter. This emulsion can record theinterference patterns between, for example, two 6,328A waves from ahelium-neon laser, at a wide range of angles of interference. Thus,these plates are extremely versatile.

While the second focusing means 110 has been described as shown in thefigure, it is evident to one skilled in the art that other means may beutilized. It is, of course, important that the object 17 be outside ofthe diverging reference beam 6. Similarly, the mirror 4, lens 5, andpinhole 8, may generally be delineated as a first focusing means 118, asother means are available to one skilled in the art.

Thus, utilizing the apparatus of FIG. 1, in the mode shown, a hologramis made from a diverging reference beam and diffused object beam.Reconstruction of the hologram, now designated by the numeral 9 todistinguish it from the undeveloped hologram (exposed photograph plate)7, is shown in F IG. 2.

FIG. 2 maintains the same first focusing means 18 as shown in FIG. l.The divergent reference beam 6 used for reconstructing the hologram isthe same as the original divergent reference beam used in constructingthe hologram. The origin point of the diverging beam will be designatedat 20, at which position the pin hole 8 is placed. An intercept means isplaced at the position 21, for the purpose of blocking out that part ofthe system comprising the second focusing means, shown in N6. 1. Thisintercept means may be a deflecting mirror, an absorbing material, orany means to prevent light from entering the initial second focusingmeans 10. On the alternative, second focusing means may be swung out ofthe system entirely. The original master mask holding means used forholding object 17 is now replaced with an object holding means 22, uponwhich reconstructed holograms will be focused. The object holding meansmay be a conveyor assembly with alignment means, or other means,including a photographic plate holder, for example. Also included wouldbe state of the art wafer-holding means, such as vacuum means. Thisobject holding means 2.2 can be designed to accept integrated circuitwafer having a photoresist covering, for the purposes of exposure to thereconstructed hologram.

A key element in the system is concave spherical mirror 23, which can bemoved into position by a mirror interposing means, such as the holdingand aligning means 24. An essential requirement of both the mirror, andthe mirror interposing means which will align the mirror, is that themirror be positioned such that the radius of curvature of sphericalmirror 23 is coincident with the origin 20 of the diverging beam oflight 6.

Thus, in operating in reconstruction mode, diverging light passingthrough hologram 9 will be reflected from mirror 23 as converging lightback through the hologram 9, to be reconstructed in focus onobject-holding means 22, which is coincident with the original objectplane 17 from which the hologram 9 was made. A viewer positioned asshown at 25 will be able to see the virtual image of the hologram. Thus,the conjugate reconstruction or read out beam has been made withoutaltering the original reference beam apparatus used for constructing thehologram. The spherical mirror 23, is adjusted such that the originalreference beam 6 is reflected by the spherical mirror, forming the imageof the pin hole 8 on itself: the center of the radius of the mirror 23coincides with the pinhole. When placing the hologram 9 in the exactsame position in which the hologram was constructed, as shown in FIG. 1,and using the original diverging reference beam 6, the DC term will passthrough the hologram 9 and be reflected by the spherical mirror 23.Because of the position of the spherical mirror 23 relative to thepinhole 20, the diverging beam will be returned along its original pathand when passing through the hologram 9 act as a convergingreconstruction beam forming a real image at the original object planell7. The incoming diverging beam 6 diffracted by the hologram 9 willform the virtual image.

Thus, in sum, by use of interposing means 21, the second focusing meansis eliminated from the system; object holding means 22 replaces mastermask holding means 17; and spherical mirror 23 is interposed into thediverging beam by mirror interposer means, while the photographic plate7 is replaced by the developed hologram 9. This system thus inherentlyassures that the original master plate will be faithfully reproduced inits original size and position, for a great ease of exposure ofphotoresist upon integrated circuit chips, as one application. Thesystem utilizes a minimum of lenses, none of which are expensive astheir size is small. Small, inexpensive, quality lenses are readilyavailable. The mirrors utilized are also readily available, as is thebeam splitter. The individual components are well known in the art.

As mentioned previously, it is desirable to eliminate as much glare orfogging from interfering with the reconstructed image that will appearat 22 in FIG. 2. A certain amount of glare, by reflection from the frontsurface of the hologram, will occur from the diverging beam 6 strikingthe glass plate hologram 9. While other holographic materials may beused other than glass, such as photographic film, nonetheless a certainamount of glare will be present. This glare can be eliminated by the useof the system shown in FIG. 3. As shown in the figure, a liquid gatemeans is used such that the reconstruction beam reflection on its firstpass to the hologram returns along its original path to the origin ofthe reference beam, thereby not interfering with the real imageprojection. This is done as follows. The original diverging referencebeam 6, used for reconstructing the hologram 9 onto the image plane 17,passes through a spherical transparent surface 30, whose front surfacehas the property of and is so aligned that its radius of curvature iscoincident with the origin point 20. The second surface 31 may be of anyshape. The spherical front surface mirror 23 is, as before, placed at adistance from the origin point 20 and is so aligned so its center ofcurvature radius also coincides with the pinhole 8 for origin point 20.The space between the mirror 23 and the rear surface 31 of the sphericaltransparent surface is suitably sealed and filled with a liquid 32 whoseindex of refraction is chosen to match that of the material 33constituting the spherical transparent surface. The hologram 9, asbefore, is inserted in a position such that it duplicates its originalposition as shown in FIG. I.

Briefly stated then, the method of reconstructing the hologram onto theoriginal object plane from which the hologram was constructed comprisesthe steps of placing the hologram in the exact original positionoccupied by the photosensitive surface which upon development became thehologram; illuminating the front face of the hologram with a divergingreference beam identical to the original diverging reference beam usedin exposing the photosensitive surface which upon development became thehologram; and finally, placing a spherical mirror opposite the backfaceof the hologram to reflect the diverging reference beam passing throughthe hologram back through the hologram as a converging beam, where thespherical mirror has the property of and is so aligned such that theradius of curvature of the spherical mirror is coincident with theorigin point of the diverging reference beam. Of course, to eliminatethe glare, an additional step can be utilized, that of placing a liquidgate between the light source and the hologram. This is done by placinga spherical transparent surface between the hologram and the divergingbeam of light, where the spherical transparent surface has the propertyof and is aligned with the diverging beam of light such that the radiusof curvature of that side 30 of the spherical transparent surfacenearest the diverging beam of light, the front face, is coincident withthe origin 20 of the diverging beam of light, and the sphericaltransparent surface being of a size sufficient to intercept that area oflight that passes through the hologram. Of course, it is necessary thatthe space between the rear surface 31 of the spherical transparentsurface, and the front surface of the spherical mirror 23 be filled witha medium having the same index of refraction as the sphericaltransparent surface.

The apparatus in practicing this method is simple to construct. Startingwith a basic reference frame, which can be a flat surface, a verticalsurface, or literally any black box," it is only necessary that a beamsplitting means and a first focusing means be located along the framecausing the incoming reference beam to become a diverging referencebeam; a means be provided for holding the spherical mirror in thedesired place, discussed previously, and that hologram aligning means belocated, which means is coincident with the position of the initialphotographic plate. More generally, of course, if it is not essentialthat the hologram be reconstructed onto its original object plane, therequirement that the hologram be positioned in the original positionfrom which the hologram was made is not necessary. A standardphotographic plate or film holder may be utilized as theholographic-photographic holding means. Of course, for the completeapparatus as shown in FIGS. I and 2, beam-splitting means may beattached to the frame, interposing means may be utilized to eliminatethe second focusing system, and may be attached to the frame. Secondfocusing means may be similarly attached to the frame, as may also bethe mirror interposing means and the real image view means.

While the above describes the preferred embodiment of this invention, inthat expensive lenses of large numerical aperture are not needed in thereconstruction mode, the basic idea of reconstructing the hologram onthe original object plane from which the hologram was constructed may beimplemented by two other means. Both of these means have thedisadvantage, compared to the preferred embodiment, of requiring qualitylenses. However, for various applications, this may not be important, orit may even be desirable to use either of the two systems disclosedbelow.

FIG. 4 shows an embodiment of this invention, utilizing a convergentreference beam in the construction mode of the hologram. The secondfocusing means, generally delineated by the numeral 10, is as describedin conjunction with the preferred embodiment of FIG. I, and will not befurther described. The reference numerals are the same as in FIG. I. Thereference beam 3 from the beam splitter 2 enters the first focusingmeans for this embodiment, generally designated by the numeral 40. Asshown, this essentially comprises mirror 39, lens 41, pinhole means 42,and converging lens 43. The incoming light enters the first focusingmeans and via the lenses emerges as a converging beam 44 to pass throughthe photographic plate 45. In conjunction with the diffused beam fromthe second focusing means 10, via diffuser 16 through the object ormaster mask 17, the photographic plate 45 is exposed. Upon development,photographic plate 45 becomes the hologram 46 as shown in FIG. 5. InFIG. 5, interposing means described previously is utilized to block outthe second focusing means 10 from the system. Object-holding means isinterposed in place of the original object 17, as described previously.First focusing means 40 remains as before, shown in FIG. 4. The hologram46 is interposed in the converging beam 44, in the exact positionoccupied by the photographic plate which upon development became thehologram. Front face reflecting convex spherical mirror 47 is nowplaced, for the reconstruction mode, in the converging beam behind thebackface of the hologram 46. Convex spherical mirror 47 is so aligned,by aligning means 48, and has the property of having a radius ofcurvature coincident with the focal point 49 of the lens 43. Whenaligned in this manner, convex spherical mirror 47 will reflect theconverging beam 44 back upon itself, resulting in the hologram 46 beingreconstructed upon the object holding means 22. Thus, again, convergingbeam 44 via lens 43 is refocused on the pinhole 42, as with thepreferred system ofFIGS. land 2.

FIG. 6 shows an apparatus for the construction of a hologram utilizingthe second focusing means 10 as described in conjunction with the priorembodiments. First focusing means 60 here comprises mirror 59, lens 61,pinhole 62, and lens 63 to create a parallel beam 64 to impinge upon thephotographic plate 65. The photographic plate 65, exposed to theparallel reference beam.64 and the beam emerging fromthe object ormaster mask 17 via the diffuser 16, will interact to expose thephotographic plate. Photographic plate 65, which upon developmentbecomes hologram 66, as shown in FIG. 7, is

relocated in the exact position which it occupied when the hologram wasexposed. Second focusing means 10 is eliminated from the system asdiscussed in conjunction with FIG. 2. The object 17 is replaced withobject holding means 2. First focusing means 60 remains as it was forthe construction mode. However, plane mirror 67 is now aligned in thereference beam 64, to reflect the beam directly back upon itself, andthus, via lens 63, refocused back upon the pinhole 62. In this manner,the reference beam 64 passing through the hologram 66 will be reflectedback upon itself by plane mirror 67, whereby the hologram will bereconstructed upon the original object plane, represented by the realimage view means 22.

Mirror 67 may be pivotally mounted to be swung into position by planemirror holding means 68. It is, of course, essential that the mirror 67be so aligned as to be normal to the incident beam 64, so as to reflectit back upon itself.

From the various embodiments shown, it is evident that the keyrequirements for reconstructing the hologram back upon the originalobject plane from which it was created, is that the original referencebeam must be utilized, or recreated for the reconstruction mode, andmust be reflected back upon itself and thus through the hologram, whichis positioned in the same position as the photographic plate from whichthe hologram upon development was formed. Broadly stated, the referencebeam must be reflected back upon itself, or when the pinhole is used,back upon the pinhole, thereby creating a true conjugate beam. Thisassures the reconstruction upon the original object plane.

The embodiments of FIG. 4 and 6 both utilize large lenses 43 and 63. Asstated previously, however, this may now be exceptionallydisadvantageous for certain applications. The preferred embodiment,however, is that shown in FIGS. 1 and 2.

As with the apparatus described in H68. 1 and 2, the apparatus of FIGS.4, 5, 6, 7, is easily constructed using known methods and knownequipment. Thus, various hinge means are available for interposing theblocking means for second focusing system it) or to move the systemphysically out of the way of the reconstruction mode. Hinge means andother alignmerit means are available for interposing the necessarymirrors during reconstruction, and this interposing means can be coupledwith alignment means to adjust and position the object-holding means 22.In this manner, the mirror and objectholding means operate together,while concurrently eliminating from the system the second focusing means10. These systems may be easily built in any convenient reference frame.As with the embodiment of FIG. 2, the glare elimination apparatus may beutilized to eliminate glare in the reconstruction system of P165. 5 and7.

Thus, the objects of this invention have been met by the method andapparatus disclosed. High-resolution holographic reconstruction can bemade to occur in the original image plane used in making the originalhologram. inexpensive lens systems may be utilized, and glare may beeliminated from the hologram. This holographic reconstruction system maybe used in the manufacture of microelectronic circuit devices, and hasthe great advantages of never being damaged by contact with thephotoresisted surfaces, minimal handling, and the great inherentadvantage that should any part of the hologram be destroyed, the balanceof the hologram nonetheless contains all the information contained onthe entire hologram in its undestroyed state. The physical removal ofthe master, now being the hologram, from the image plane allows veryrapid interchanging of parts to be exposed, in the photoresist process.

The glare removal system also disclosed by requiring only that theliquid have the same refractive index as the glass or material used forthe spherical transparent surface, allows a great variety of materialsto be used. One material which has given good results is the liquiddecahydronapthalene, in conjunction with optical glass used for thespherical transparent surface. Other materials may also be utilized.

While particular embodiments have been shown, other uses and variationsof that disclosed will be evident to those skilled in the art, withoutdeparting from the spirit and scope of this invention.

What is claimed is:

1. A method of reconstructing a holographic image comprising:

illuminating the front face ofa hologram constructed with a firstdiverging reference beam with a second diverging reference beam;

placing a spherical mirror opposite the backface of said hologram, saidspherical mirror having the property of being aligned with said seconddiverging beam of light such that the radius of curvature of saidspherical mirror is coincident with the origin of said second divergingbeam oflight;

whereby diverging light passing through said hologram is reflected backfrom said spherical mirror as a converging beam back through saidhologram, reconstructing the image from which said hologram wasconstructed.

2. A method of reconstructing a glare-free holographic image comprising:

illuminating the front face of a hologram constructed with a firstdiverging reference beam with a second diverging reference beam;

placing a spherical mirror opposite the backface of said hologram, saidspherical mirror having the property of and being aligned with saidsecond diverging beam of light such that the radius of curvature of saidspherical mirror is coincident with the origin of said second divergingbeam of light;

placing a liquid gate means between said hologram and the origin of saiddiverging beam of light, comprising a spherical transparent surfacehaving the property of and being aligned with said diverging beam oflight such that the radius of curvature of that side of said sphericaltransparent surface nearest said origin point of said diverging beam oflight is coincident with the origin of said diverg ing beam of light,said spherical transparent surface being of a size sufficient tointercept that area of light that passes through said hologram;

filling the space between said spherical transparent surface and saidspherical mirror with a medium having the same index of refraction assaid spherical transparent surface;

whereby light reflected from said spherical mirror back through saidhologram reconstructs a glare-free real image from which said hologramwas constructed.

3. The method of claim 1 wherein said spherical mirror is of a size andis so located as to reflect back through said holo gram the entire areaof light originally passing through said hologram to said sphericalmirror.

4. The method of claim 2 wherein said spherical mirror is of a size andis so located as to reflect back through said hologram the entire areaof light originally passing through said hologram to said sphericalmirror.

5. Apparatus for the reconstruction of a hologram comprising:

a reference frame;

first focusing means located along said frame for causing an incomingreference beam to become a diverging reference beam;

a spherical mirror located along said frame, said spherical mirrorhaving the property of and being aligned with said diverging referencebeam such that the radius of curvature of said spherical mirror iscoincident with the origin point of said diverging reference beam; and

hologram-aligning means located along said frame for inter posing ahologram between said diverging means and said spherical mirror, toreflect that part of said reference beam passing through the hologramback from said spherical mirror, through the hologram, as a conjugatebeam.

6. The apparatus of claim 5 including a liquid gate means located uponsaid frame between the origin point of said diverging reference beam andsaid hologram aligning means, comprising a spherical transparent surfacehaving the property of and being aligned with said diverging referencebeam such that the radius of curvature of said spherical transparentsurface nearest the origin point is coincident with the origin point,said spherical transparent surface being of a size sufficient tointercept that area of light that passes through the hologram, andcontaining means for containing a medium having the same index ofrefraction as said spherical transparent surface, located between andcontacting at least said spherical transparent surface and saidspherical mirror such that said spherical transparent surface and saidspherical mirror comprise at least two of the containing walls of saidcontaining means.

7. The apparatus of claim 6 wherein a medium of the same index ofrefraction as said spherical transparent surface is located within saidcontaining means.

8. The apparatus of claim 5 including object holding means located atthe real image plane of the reconstructed hologram for allowing the realimage to be reconstructed thereon.

9. Apparatus for the construction and reconstruction of hologramscomprising:

a reference frame;

a beam-splitting means located upon said frame to split an incoming beamof light to first and second focusing means located upon said frame,said first focusing means comprising means for converting said incomingbeam of light to a diverging beam of light capable of illuminating aphotographic recording surface located within the diverging beam oflight by a photographic-holographic holding means attached to saidframe;

said second focusing means comprising means for illuminating the objectto be holographed such that the diverging image of said object iscapable of falling upon the photo graphic recording surface located bysaid photographicholographic holding means, the object being located bymaster mask-holding means attached to said frame, the object plane beinglocated outside of the diverging beam of light created by said firstfocusing means;

intercept means attached to said frame to intercept and cut off incominglight to said second focusing means;

a spherical mirror attached to mirror interposing means attached to saidframe to interpose said spherical mirror at a position after saidphotographic-holographic holding 10 means relative to the origin of thediverging beam of light, said spherical mirror having the property ofand said mirror interposin ..:ans so aligned that when said mirror isinterposed, its radius of curvature is coincident with the origin ofsaid diverging light; and

object-holding means located upon said frame ,to be interchangeable withsaid master masking-holding means, to allow a reconstructed hologramfrom a hologram originally made when said first and second focusingmeans and a photographic recording surface are utilized, to bereconstructed upon a viewing surface located by said object holdingmeans when the photographic plate is developed into a hologram andpositioned in said photographic-holographic holding means, saidspherical mirror is interposed, and said intercept means are utilized.

10. The apparatus of claim 9 including liquid gate means, said liquidgate means being interposed into said diverging reference beam when saidspherical mirror is interposed into said diverging reference beam.

11. A method of reconstructing a hologram onto the original object planefrom which said hologram was constructed, comprising the steps of:

placing said hologram in the original position occupied by thephotosensitive surface which upon development became said hologram;

illuminating the front face of said hologram with a diverging referencebeam identical to the original diverging reference beam used in exposingthe photosensitive surface which upon development became said hologram;and

placing a spherical mirror opposite the backface of said hologram toreflect said diverging reference beam passing iii) through said hologramback through said hologram as a converging beam, said spherical mirrorhaving the property of and being aligned such that the radius ofcurvature of said spherical mirror is coincident with the origin pointof said diverging reference beam.

12. A method of reconstructing a hologram onto the original object planefrom which said hologram was constructed, comprising the steps of:

placing said hologram in the original position occupied by thephotosensitive surface which upon development became said hologram;

illuminating the front face of said hologram with a reference beamidentical to the original convergent reference beam used in exposing thephotosensitive surface which upon development became said hologram; and

placing a convex spherical mirror opposite the backface of saidhologram, said mirror having the property of and being so aligned thatthe radius of curvature of said mirror is coincident with the focalpoint of said convergent reference beam so to reflect said referencebeam back upon itself and thus back through said hologram as a conjugatebeam;

whereby said hologram is reconstructed upon the original object planefrom which hologram was constructed.

13. A method of reconstructing a holographic image comprising:

illuminating the front face of a hologram constructed with a firstconvergent reference beam with a second reference beam;

placing a reflecting means opposite the backface of said hologram,wherein said reflecting means is a convex spherical mirror having theproperty of and being so aligned that the radius of curvature of saidmirror is coincident with the focal point of said convergent referencebeam so as to reflect said second reference beam back upon itself andthus back through said hologram as a conjugate beam;

whereby said second reference beam is reflected from said reflectingmeans back through said hologram, reconstructing the image from whichsaid hologram was constructed.

1. A method of reconstructing a holographic image comprising:illuminating the front face of a hologram constructed with a firstdiverging reference beam with a second diverging reference beam; placinga spherical mirror opposite the backface of said hologram, saidspherical mirror having the property of being aligned with said seconddiverging beam of light such that the radius of curvature of saidspherical mirror is coincident with the origin of said second divergingbeam of light; whereby diverging light passing through said hologram isreflected back from said spherical mirror as a converging beam backthrough said hologram, reconstructing the image from which said hologramwas constructed.
 2. A method of reconstructing a glare-free holographicimage comprising: illuminating the front face of a hologram constructedwith a first diverging reference beam with a second diverging referencebeam; placing a spherical mirror opposite the backface of said hologram,said spherical mirror having the property of and being aligned with saidsecond diverging beam of light such that the radius of curvature of saidspherical mirror is coincident with the origin of said second divergingbeam of light; placing a liquid gate means between said hologram and theorigin of said diverging beam of light, comprising a sphericaltransparent surface having the property of and being aligned with saiddiverging beam of light such that the radius of curvature of that sideof said spherical transparent surface nearest said origin point of saiddiverging beam of light is coincident with the origin of said divergingbeam of light, said spherical transparent surface being of a sizesufficient to intercept that area of light that passes through saidhologram; filling the space between said spherical transparent surfaceand said spherical mirror with a medium having the same index ofrefraction as said spherical transparent surface; whereby lightreflected from said spherical mirror back through said hologramreconstructs a glare-free real image from which said hologram wasconstructed.
 3. The method of claim 1 wherein said spherical mirror isof a size and is so located as to reflect back through said hologram theentire area of light originally passing through said hologram to saidspherical mirror.
 4. The method of claim 2 wherein said spherical mirroris of a size and is so located as to reflect back through said hologramthe entire area of light originally passing through said hologram tosaid spherical mirror.
 5. Apparatus for the reconstruction of a hologramcomprising: a reference frame; first focusing means located along saidframe for causing an incoming reference beam to become a divergingreference beam; a spherical mirror located along said frame, saidspherical mirror having the property of and being aligned with saiddiverging reference beam such that the radius of curvature of saidspherical mirror is coincident with the origin point of said divergingreference beam; and hologram-aligning means located along said frame forinterposing a hologram between said diverging means and said sphericalmirror, to reflect that part of said reference beam passing through thehologram back from said spherical mirror, through the hologram, as aconjugate beam.
 6. The apparatus of claim 5 including a liquid gatemeans located upon said frame between the origin point of said divergingreference beam and said hologram aligning means, comprising a sphericaltransparent surface having the property of and being aligned with saiddiverging reference beam such that the radius of curvature of saidspherical transparent surface neareSt the origin point is coincidentwith the origin point, said spherical transparent surface being of asize sufficient to intercept that area of light that passes through thehologram, and containing means for containing a medium having the sameindex of refraction as said spherical transparent surface, locatedbetween and contacting at least said spherical transparent surface andsaid spherical mirror such that said spherical transparent surface andsaid spherical mirror comprise at least two of the containing walls ofsaid containing means.
 7. The apparatus of claim 6 wherein a medium ofthe same index of refraction as said spherical transparent surface islocated within said containing means.
 8. The apparatus of claim 5including object holding means located at the real image plane of thereconstructed hologram for allowing the real image to be reconstructedthereon.
 9. Apparatus for the construction and reconstruction ofholograms comprising: a reference frame; a beam-splitting means locatedupon said frame to split an incoming beam of light to first and secondfocusing means located upon said frame, said first focusing meanscomprising means for converting said incoming beam of light to adiverging beam of light capable of illuminating a photographic recordingsurface located within the diverging beam of light by aphotographic-holographic holding means attached to said frame; saidsecond focusing means comprising means for illuminating the object to beholographed such that the diverging image of said object is capable offalling upon the photographic recording surface located by saidphotographic-holographic holding means, the object being located bymaster mask-holding means attached to said frame, the object plane beinglocated outside of the diverging beam of light created by said firstfocusing means; intercept means attached to said frame to intercept andcut off incoming light to said second focusing means; a spherical mirrorattached to mirror interposing means attached to said frame to interposesaid spherical mirror at a position after said photographic-holographicholding means relative to the origin of the diverging beam of light,said spherical mirror having the property of and said mirror interposingmeans so aligned that when said mirror is interposed, its radius ofcurvature is coincident with the origin of said diverging light; andobject-holding means located upon said frame to be interchangeable withsaid master masking-holding means, to allow a reconstructed hologramfrom a hologram originally made when said first and second focusingmeans and a photographic recording surface are utilized, to bereconstructed upon a viewing surface located by said object holdingmeans when the photographic plate is developed into a hologram andpositioned in said photographic-holographic holding means, saidspherical mirror is interposed, and said intercept means are utilized.10. The apparatus of claim 9 including liquid gate means, said liquidgate means being interposed into said diverging reference beam when saidspherical mirror is interposed into said diverging reference beam.
 11. Amethod of reconstructing a hologram onto the original object plane fromwhich said hologram was constructed, comprising the steps of: placingsaid hologram in the original position occupied by the photosensitivesurface which upon development became said hologram; illuminating thefront face of said hologram with a diverging reference beam identical tothe original diverging reference beam used in exposing thephotosensitive surface which upon development became said hologram; andplacing a spherical mirror opposite the backface of said hologram toreflect said diverging reference beam passing through said hologram backthrough said hologram as a converging beam, said spherical mirror havingthe property of and being aligned such that the radius of curvature ofsaid spherical mirror is coincident with the origin point of sAiddiverging reference beam.
 12. A method of reconstructing a hologram ontothe original object plane from which said hologram was constructed,comprising the steps of: placing said hologram in the original positionoccupied by the photosensitive surface which upon development becamesaid hologram; illuminating the front face of said hologram with areference beam identical to the original convergent reference beam usedin exposing the photosensitive surface which upon development becamesaid hologram; and placing a convex spherical mirror opposite thebackface of said hologram, said mirror having the property of and beingso aligned that the radius of curvature of said mirror is coincidentwith the focal point of said convergent reference beam so to reflectsaid reference beam back upon itself and thus back through said hologramas a conjugate beam; whereby said hologram is reconstructed upon theoriginal object plane from which hologram was constructed.
 13. A methodof reconstructing a holographic image comprising: illuminating the frontface of a hologram constructed with a first convergent reference beamwith a second reference beam; placing a reflecting means opposite thebackface of said hologram, wherein said reflecting means is a convexspherical mirror having the property of and being so aligned that theradius of curvature of said mirror is coincident with the focal point ofsaid convergent reference beam so as to reflect said second referencebeam back upon itself and thus back through said hologram as a conjugatebeam; whereby said second reference beam is reflected from saidreflecting means back through said hologram, reconstructing the imagefrom which said hologram was constructed.